Embodiments relate generally to marine vibrators for marine geophysical surveys. More particularly, embodiments relate to addition of a variable mass load to the outer shell of a marine vibrator to compensate for air spring effects.
Sound sources are generally devices that generate acoustic energy. One use of sound sources is in marine seismic surveying in which the sound sources may be employed to generate acoustic energy that travels downwardly through water and into subsurface rock. After interacting with the subsurface rock, e.g., at boundaries between different subsurface layers, some of the acoustic energy may be returned toward the water surface and detected by specialized sensors. The detected energy may be used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
Most of the sound sources employed today in marine seismic surveying are of the impulsive type, in which efforts are made to generate as much energy as possible during as short a time span as possible. The most commonly used of these impulsive-type sources are air guns that typically utilize compressed air to generate a sound wave. Other examples of impulsive-type sources include explosives and weight-drop impulse sources. Another type of sound source that can be used in seismic surveying includes marine vibrators, such as hydraulically powered sources, electro-mechanical vibrators, electrical marine seismic vibrators, and sources employing piezoelectric or magnetostrictive material. Vibrator sources typically generate vibrations through a range of frequencies in a pattern known as a “sweep” or “chirp.”
Prior sound sources for use in marine seismic surveying have typically been designed for relatively high-frequency operation (e.g., above 10 Hz). However, it is well known that as sound waves travel through water and through subsurface geological structures, higher frequency sound waves may be attenuated more rapidly than lower frequency sound waves, and consequently, lower frequency sound waves can be transmitted over longer distances through water and geological structures than higher frequency sound waves. Thus, efforts have been undertaken to develop sound sources that can operate at low frequencies.
Marine vibrators have been developed that may have least one resonance frequency of about 10 Hz or lower. In order to achieve a given level of output in the water, these marine vibrators typically need to undergo a change in volume. In order to work at depth while minimizing structural weight, the marine vibrator may be pressure balanced with external hydrostatic pressure. As the internal gas (e.g., air) in the source increases in pressure, the bulk-modulus (stiffness) of the internal gas also rises. This increase in bulk-modulus or “air spring” of the internal gas tends to be a function of the operating depth of the source. Further, the stiffness of the structure and the internal gas are primary determining factors in the source's resonance frequency. Accordingly, the resonance of the marine vibrator may undesirable increase when the vibrator is towed at depth, especially in vibrators where the interior volume of the source may be pressure balanced with the external hydrostatic pressure.